1. Field of the Invention
The subject invention relates to pressure transducers and, more specifically, to methods for making differential pressure transducers for corrosive or conductive fluids by electrostatic bonding, and to electrostatically bonded pressure transducers for corrosive or conductive fluids.
2. Information Disclosure Statement
The following disclosure statement is made pursuant to the duty of disclosure imposed by law and formulated in 37 CFR 1.56(a). No representation is hereby made that information thus disclosed in fact constitutes prior art, inasmuch as 37 CFR 1.56(a) relies on a materiality concept which depends on uncertain and inevitably subjective elements of substantial likelihood and reasonableness and inasmuch as a growing attitude appears to require citation of material which might lead to a discovery of pertinent material though not necessarily being of itself pertinent. Also, the following comments contain conclusions and observations which have only been drawn or become apparent after conception of the subject invention or which contrast the subject invention or its merits against the background of developments which may be subsequent in time or priority.
As mentioned in U.S. Pat. No. 3,505,634, by G. Von Vick, issued Apr. 7, 1970 for differential pressure transducer, the prior art has resorted to the use of two different transducers, or has required a double bellows or other relatively complex arrangement for differential pressure measurements involving two corrosive media. Conventional practice for providing media isolation of the electrically active side of strain gage pressure transducers required installation of a compliant isolating diaphragm, with oil or another dielectric fluid, acting as a pressure transfer medium and filling the cavity between the isolating diaphragm and the side of the measuring diaphragm carrying the electrical strain gages. For transducers in which the measuring diaphragm is silicon or another semiconductor, with integral diffused strain gages, it is also necessary to electrically isolate the inactive side of the diaphragm from electrically conductive fluids. Use of isolator diaphragms with oil fill is conventional practice here also.
The isolator diaphragm/oil fill approach is expensive, adds error to measurement, due to the characteristics of the isolator diaphragm or diaphragms, limits environmental operating temperature range, and adds moving system mass or masses, making such transducers highly sensitive to vibration, acceleration and orientation.
The above mentioned Von Vick patent proposes protection from the media by covering layers comprising organic or ceramic material that is inert to the media, as therein described. The bilaterally covered diaphragm is located between tubular members defining opposite pressure ports and having lateral flanges connected by bolts or fasteners to an annular thickened body at the periphery of the diaphragm.
U.S. Pat. No. 3,662,312, by W. Thorp et al, issued May 9, 1972, discloses semiconductor strain transducers in which a semiconductor wafer is bonded to a rigid metal supporting ring which, in turn, is bonded to an annular insulating member secured to the transducer housing. An electrically insulating layer of silicon dioxide is deposited on the non-resistor bearing major wafer face to prevent current leakage in the presence of a conductive medium whose pressure is to be measured. A metallic coating is provided on the silicone dioxide layer so as to make good electrical contact with the metal housing, thereby removing interference from radio frequency or other alternating current flow.
U.S. Pat. Nos. 3,753,196, issued Aug. 14, 1973, and 3,819,431, issued June 25, 1974, by Kurtz et al, disclose transducers employing integral protective coatings and support and methods for making same. A silicon diaphragm is mounted in a housing, with piezo-resistive sensing elements facing away from the applied force surface having formed thereon a layer of silicon dioxide which serves to protect the diaphragm against deleterious agents present in the force transmitting environment, while further serving to eliminate an undesirable bimetallic effect. Kurtz et al expressly prefer that to prior-art approaches in which the silicon diaphragm was coated with a grease-like compound or an epoxy, or in which protective layers of gold, platinum or other metal, rubber or the like were bonded or glued to the diaphragm. Kurtz et al expressly designated such approaches as unreliable and expensive.
U.S. Pat. No. 4,314,226, by T. Oguro et al, issued Feb. 2, 1982, shows a prior-art type of silicon diaphragm having a film of silicon dioxide formed thereon and of the fused resistors, and aluminum leads formed on that film and electrically connected to these resistors. Oguro et al mentioned that such an arrangement is not adequate for differential pressure transducers, and proposed an approach in which a protective layer, including a silicon epitaxial layer, opposite in conductive type to the resistors, is formed thereon and on the silicon base, followed by the formation of an electrically insulating silicon dioxide layer on the silicon epitaxial layer.
U.S. Pat. No. 4,411,158, by A. Schaff, Jr., issued Oct. 25, 1983 for apparatus for sensing the condition of a fluid, discloses a semiconductor chip having a diaphragm portion adapted for exposure to pressurized fluid and an adjacent terminal portion adapted for connection to external circuitry for providing a measurement of the condition of the fluid. The semiconductor chip has sensing elements formed within the diaphragm portion and conductor elements in electrical contact with the sensing elements formed within and extending from the diaphragm portion to the terminal portion. The diffusion process, by which strain gage resistors are diffused into the basic silicon wafer or chip by conventional planar diffusion or ion implantation, includes the growing of a protective layer of silicon dioxide over the diffused or implanted areas. Conventional cement is employed for providing a fluid barrier isolating the diaphragm portion from the terminal portion. Opposite high and low pressure fluid ports are provided by matching cartridge halves which are cemented to each other.
For electrostatic bonding, reference may be had to U.S. Pat. Nos. 3,397,278, by D. I. Pomerantz, issued Aug. 13, 1968, for Anodic Bonding, 3,417,459 and 3,470,348 by D.I. Pomerantz et al, issued Dec. 24, 1968 and Sept. 30, 1969 for Bonding Electrically Conductive Metals to Insulators, and for Anodic Bonding of Liquid Metals to Insulators, respectively. Briefly, an electrically conductive element is bonded to an insulator element by placing the elements in close surface contact, heating the insulator element to render it electrically conductive, and applying a potential across the elements thereby creating an electrostatic field for bonding the elements to each other.
Conventionally, strain gages are mounted by means of epoxy. Obvious problems are a tendency of the epoxy to creep under repeated applications of stress and the limited temperature range of the epoxy.